1. Field of the Invention
The present invention relates generally to a wireless cellular communication system, and more particularly, to an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system in which resource allocation is made on per resource block basis.
2. Description of the Related Art
FIG. 1 illustrates a reference signal pattern defined in 3GPP LTE Release 8, where Reference Elements (REs) 0-3 are Cell-specific Reference Signals (CRS) for antenna ports 0-3, respectively. REs 5 are for User Equipment (UE)-specific Demodulation Reference Signals (DMRS) indicated as virtual port 5.
The CRS uses common reference sequences known to all UEs, while the DMRS is precoded according the antenna weights applied to corresponding data payloads.
Regular Reference Signal (RS)-RE distribution for simpler channel estimator design preferably includes uniform spacing of RS-REs.
Identical RS-RE distribution for multiple Resource Blocks (RBs) is also preferable for both simpler transmitter and receiver designs.
Joint RS decoding is performed across contiguous RBs for improved estimation performance. The channel estimation performance can be significantly improved by basing on more neighboring RS-REs. RS-RE placement at or near the edge of RB allocation allows for better interpolation. Channel estimation by extrapolation is worse than that using interpolation. Thus, RS-RE placement at the edge of resource allocation is preferable, especially in the frequency domain.
However, since it is difficult to meet all the above requirements, a good tradeoff/compromise must be made among them.
FIG. 2 illustrates DMRS designs for one RB. P1-P2 are DMRS patterns for one-layer transmission. In total, 12 REs are dedicated for DMRS purpose. In P1, RS-REs are connected to one another in the frequency domain, while in P2, RS-REs are connected to one another in the time domain. RS-REs are placed at the RB edge for better channel estimation performance.
P3-P5 are DMRS patterns for two-layer transmission. In total, 12 REs are dedicated for DMRS purpose. In P3 and P4, RS-REs are divided for layer one and layer two transmission based on P1 and P2, respectively. In P4, the layer-one and layer-two transmissions reuse the same RS-REs, and orthogonal codes are applied to each layer (Code Division Multiplexing (CDM)).
P6-P8 are DMRS patterns for four-layer transmission. In total 24, REs are dedicated for DMRS purpose. In P6, 24 RS-REs are divided so that each layer occupies 6 REs. In P7 and P8, the layer-one and layer-two transmissions reuse the same RS-REs, while the layer-one and layer-two transmissions reuse another set of RS-REs.
P9 DMRS patterns are designed for multi-layer transmission up to eight layers. All the layers reuse the same 24 REs, and orthogonal cover codes are applied to distinguish one layer from another.
All P1-P9 designs attempt to place RS-REs at the frequency edge of an RB. This is preferable when there is only one stand-alone RB allocation for a UE. However, when the resource allocation to a certain UE is contiguous, i.e., multiple contiguous RBs across the frequency domain are assigned to the UE, those one-RB-optimized RS patterns may no longer be preferable. FIG. 3 illustrates an example by concatenating multiple RBs with P7 design. RS-REs aggregate at the border of two RBs. However, this is not an efficient design, since RSs that are close to each other may not provide a significant amount of gain comparing when compared to a design with uniform RS spacing given the same overall RS density.